One of the best — improves productivity and clears clutter! Rated 5 out of 5 stars
I've long been an advocate of vertical tabs in Firefox, previously editing userChrome.css and using some other add-ons, which didn't have FF3 versions ready when I upgraded. So I found Tree Style Tab, and since a few days ago, have been enjoying it immensely and intensely.
Vertical tabs should be built into Firefox — if you have a lot of tabs open at once, this add-on's definitely for you. Think about how long lists of stuff, like phone numbers and spreadsheets, are ordered: vertically. Same principle applies here.
Once you start opening up multiple tabs, especially on a widescreen monitor (where there's ample room for main web content to display), you'll quickly indulge in the benefits. And if you think the left vertical tab bar still takes too much space, you can easily hide or resize it — no sweat.
Furthermore, you can expand/collapse vertical tabs into trees, focusing on different topics before turning your attention to the next. This is audaciously brilliant and a perfect complement to Tab Mix Plus' extensibility (both of them work together for me), as well as the specific "Open All in Tabs" option.
If you want to save yourself a lot of time in the long run, download Tree Style Tab right now and give it a go. It's an incredible effectiveness-booster, removes the problem of hunting for squinty or ugly multi-row horizontal tabs, and overall will make you smile. LOTS. :-D